Actually, kinda hard to fault them. Everything sounds like it was setup correctly. You just stumbled upon the surprise. It sounds like they did what they could to compensate, but you blew the surprise. If your husband had picked up the note & opened it, then everything would have worked out the way it was supposed to.
Not all surprises go off as planned. It's hard to keep something like that from someone especially when you have to figure a way to get them to the place where its being held without making them suspicious. (Party, engagement, b-day, etc).
I would just be thankful for the idea & leave it at that. There's alot of women out there with husbands who won't do things like that or are stuck in an abusive relationship.